In general, a motor is provided in a compressor which is a mechanical apparatus receiving power from a power generation apparatus such as an electric motor, a turbine or the like, and compressing the air, refrigerant or various operation gases to raise a pressure. The compressor has been widely used for electric home appliances such as refrigerators and air conditioners, and application thereof has been expanded to the whole industry.
Particularly, the compressors are roughly classified into a reciprocating compressor, wherein a compression space to/from which an operation gas is sucked and discharged is defined between a piston and a cylinder, and the piston linearly reciprocates in the cylinder to compress refrigerant, a rotary compressor, wherein a compression space to/from which an operation gas is sucked and discharged is defined between an eccentrically-rotating roller and a cylinder, and the roller eccentrically rotates along an inner wall of the cylinder to compress refrigerant, and a scroll compressor, wherein a compression space to/from which an operation gas is sucked and discharged is defined between an orbiting scroll and a fixed scroll, and the orbiting scroll rotates along the fixed scroll to compress refrigerant.
Recently, among the reciprocating compressors, a linear compressor has been actively developed because it improves compression efficiency and provides simple construction by removing a mechanical loss caused by motion conversion by directly connecting a piston to a linearly-reciprocating driving motor.
FIG. 1 is a block diagram illustrating construction of a motor control apparatus applied to a conventional linear compressor.
As illustrated in FIG. 1, the motor control apparatus includes a rectification unit which is composed of a diode bridge 11 receiving input of AC power which is commercial power, and rectifying and outputting the resulting voltage, and a capacitor C1 smoothing the rectified voltage, an inverter unit 12 which receives a DC voltage, converts the DC voltage into an AC voltage according to a control signal from a control unit 17, and supplies the AC voltage to a motor unit, the motor unit which includes a motor 13 and a capacitor C2 connected in series to the motor 13, a voltage detection unit 14 which detects a both-end voltage of the capacitor C1, a current detection unit 15 which detects a current flowing through the motor unit, an operation unit 16 which operates a counter electromotive force (EMF) from the sensed voltage from the voltage detection unit 14 and the sensed current from the current detection unit 15, and the control unit 17 which generates a control signal, reflecting the counter EMF from the operation unit 16 and the sensed current from the current detection unit 15.
In the control apparatus, the operation unit 16 operates the counter EMF by the following Formula 1:
                    EMF        =                  V          -                      L            ⁢                                          ⅆ                ⅈ                                            ⅆ                l                                              -                                    1              C                        ⁢                          ∫                              ⅈ                ⁢                                  ⅆ                  t                                                              -          Ri                                    〈                  Formula          ⁢                                          ⁢          1                〉            
Here, L represents an inductance of the motor 13, V represents an applied voltage to the inverter unit 12, and R represents a resistance value of the motor 13.
That is, the operation unit 16 operates the counter EMF according to the sensed current from the current detection unit 15.
FIG. 2 is a graph showing cooling capacity modulations of the linear compressor of FIG. 1. The graph of FIG. 2 shows a control result of the control unit 17 for acquiring a necessary cooling capacity, when a BLDC inverter is applied to the inverter unit 12 of the motor control apparatus.
As a temperature which is a load rises, the control unit 17 controls the inverter unit 12 to forcibly raise an AC voltage applied to the motor 13, thereby acquiring a cooling capacity required for the load. As shown, when a temperature rises from 10□ to 50□, the control unit 17 performs four forcible voltage raising controls to thereby acquire a target cooling capacity or cooling capacity ratio.
However, when the control unit 17 acquires the cooling capacity through plural forcible voltage raising controls or forcible voltage dropping controls, the control unit 17 must perform plural controls. Reliability of components in the motor control apparatus is severely reduced due to continuous voltage modulations for the forcible voltage raising and dropping controls. In addition, a protection device (protection circuit) should be additionally provided against the plural voltage modulations.